Biocontrol Backfires

Biocontrol Backfires

Armies of venomous cane toads (Bufos marinus) threaten to overrun large parts of Australia, reports a recent study in the science journal, Nature (Volume 439, 707 A, February 2006). Scientists of the University of Sydney, who conducted this study, say that the species -- introduced into the country more than 70 years back from South America to wipe out the sugarcane-ravaging beetles -- has developed a leggier, faster-moving form that is now hopping rapidly across the continent.

More than a hundred cane toads were introduced into the northeast Australian state of Queensland in 1935. But unseasonal breeding led these amphibians to multiply to more than 60,000 in less than a year. The creatures adapted well to their new environment and spread through Queensland.

The cane toads, however, proved inept to the task for which they were introduced: the beetles were just too sprightly for the frogs. But to sate its appetite, the ambhibian began snapping up other bugs and quickly out-competed other native insect-eaters.

Several other indigenous Australian species have felt the toads' fury. When pushed into a corner, the creature secretes poison, stored in two sacs behind its head, killing off the predator in minutes. An encounter with the amphibian spells sure doom for snakes, lizards, water birds, even crocodiles and dingos.

The toads' march There aren't any predators in Australia to check toads. Consequently, the amphibians, which can grow up to 2 kg, have spread over more than a million sq km -- roughly a third of Australia. The cane toads' range in the country now includes coastal Queensland, and associated inland areas, coastal northern New South Wales and coastal Northern Territory. However, till a few years back, they were regarded as little more than irritants. Children in north and northeast Australia would squash the toads for pleasure, motorists would try and crush them under car wheels, aspiring cricketers would slam them out of the ground and weekend gardeners just chase them down with a lawn mower. But the toads attracted little scientific attention till 2001, when they showed up in Kakadu National Park -- an area of pristine wetlands and savannah forest. The invasion of this World Heritage site alarmed many. Zoologists had long regarded the cane toad as somewhat of a slow plodder. But it was time to change this view.

Scientists of the University of Sydney homed in on cane toads invading the state of Northern Territory, at a site about 60 km east of Darwin. They caught a few of these creatures, measured them and also attached a radio-transmitter, weighing about 5-6 grammes around their waists to track their movements. The researchers found that toads at the vanguard of the invasion had legs that were up to 6 per cent longer than average. "During a cane-toad invasion, the individuals at the front are there because they have moved the furthest," explained Benjamin Phillips, an author of the Nature paper. "We showed that the toads that are the first to arrive are the ones with the longest legs, and the ones to arrive last have shorter legs," Phillips added.

Evolution?
The University of Sydney researchers also concluded that the limbs of the newer populations of toads tended to be longer than those of the long-established populations in Queensland. Moreover, the scientists found that the creatures were moving incredibly quickly, around 50-60 km a year, covering distances about five times faster than when they arrived 70 years ago.

Such evolution portends an eco-nightmare, warn the University of Sydney researchers. Insects and bacteria are believed to adapt quickly to changing environment, but vertebrates evolving rapidly in a new environment is relatively unheard of, they claim. There is, however, some disagreement amongst these experts on the use of the term 'evolution' to describe the phenomenon: some of them prefer to ascribe the long-legged toads to population dynamics. These scientists argue that toads, which are the first to colonise an area are invariably the fittest and the fastest. These individuals would have eaten the most, grown the best, and therefore are able to move longer distances more quickly in search of new feeding areas. So, the motivation to move comes from competition in existing areas, and an abundance of resources (food, space) in uncolonised areas, they contend.

semantic disputes notwithstanding, the scientists agree that the march of the cane toads spells ecological change. Though no documented extinctions are attributed to the cane toads, experts agree that the creatures dramatically modify the behaviour of plants and animals in the ecosystems they invade For example, Phillips and his colleague Richard Shine have showed in an earlier paper that two snake species have developed smaller heads and can no longer eat the toads. This is one way to avoid the venomous creature. Other would-be toad predators have simply altered their diets to exclude the creature.

As scientists learn more about cane toad biology, they are likely to devise strategies to eradicate these creatures, such as changing the character of a breeding pond. Till then the toads are likely to be permanent fixtures in Australia and are sure to continue their worrying spread.